Cone Beam Computed Tomography (CBCT) is an x-ray technology that produces 3D images of the bony structure being scanned. These scanners have been available for craniofacial imaging since 1999 in Europe, 2001in the United States, and 2005 in Australia.
Although the imaging is created using computed tomography (CT) and appear similar to conventional medical multislice CT (MSCT) images, the method of x-ray emission and capture is quite different.

MSCT scanners use a single row or a series (4, 8, 12, 32 and now 64) of solid-state detectors combined with a fan shaped beam to capture the x-ray. As a result the MSCT scanner provides a set of consecutive slices of the patient. Sophisticated mathematics stacks these slices together to produce 3D imaging. Unlike 2D digital images which work with pixels, 3D imaging is created using volumes of information. In the case of MSCT scans, these volumes (voxels) are anisotropic in dimension (i.e. height, breadth and depth dimensions are different.

Cone Beam CT scanners use a pulsed cone-shaped x-ray beam, which is captured on a square 2 dimensional array of detectors. Instead of data taking the form of a set of consecutive slices as with the conventional medical CT scanners, the Cone Beam CT scanner provides a volume of data. The voxels created are isotropic (i.e. of equal height, breadth and depth dimensions). The uniform nature of the voxels means; there is superior dimensional accuracy in the reconstruction process of the raw data (DICOM - Digital Image COMmunication in medicine) captured by the scan, the data is easily uploaded into 3rd party software, and accuracy is not lost if the image data undergoes repositioning or reorientation.

How does CBCT compare with other dental x-ray imaging?

To understand the differences between the different types of dental x-rays in use, please refer to Table 1.

With any imaging that involves the irradiation of the patient, the clinician should be adopting the ALARA (As Little As Reasonably Achievable) principle. The balance between the amount and quality of the image produced and needed per unit of radiation is an important part of the ALARA principle, together with logistical and socio-economic considerations.

The differences in the methods of image acquisition between MSCT and CBCT have a substantial effect on radiation dose to the patient.

Medical CT images used for dental applications are, more often than not, taken without utilising optimum dose reducing programmes. The result is a radiation exposure in excess of 2000 (microsieverts) SV to the patient, (optimal dose reduction settings can reduce this to just over 310). From Table 1, this can be seen to equate to between 8 and 9 months of the expected daily background radiation we are constantly exposed to from space.

The CBCT machine that is used at Advanced Imaging Services (i-CAT, Imaging Sciences International, USA) subjects the patient to 34 SV for standard dental diagnostics. If more involved procedures require imaging that focuses on specific dental anatomy (e.g. nerves, sinuses etc), a maximum exposure of 136 SV may be necessary. This amounts to the equivalent of 3 to 4 days natural radiation, or 13 to 15 days at the most.

Dosages for traditional 2D dental diagnostic x-ray image acquisitions can be seen in Table 1. The important difference between traditional 2D and CBCT/MSCT 3D images should also be evaluated in terms of quantity and quality.

Image quantity and quality:

Traditional 2D x-rays give a limited amount of information presented in a pixel arrangement. Pixels can only produce images in one plane, along the x and y-axes. There is no depth dimension, or z-axis. Training coupled with experience in reading these images will allow the clinician to express an informed opinion about the anatomy presented, at best. Determining whether a nerve sits in front of, behind or, more critically, between the roots of a tooth can be difficult to determine. These images are also subject to various degrees of magnification and distortions. The degree of magnification can be accommodated due to the fact that the level of magnification is given for the images and this value can be used to formulate real measurements from measurements taken from the x-ray. Distortions cannot be computed as these errors occur due to variations of magnification in a given area. The posterior anatomy of the upper and lower jaws house critical structures such as nerves and sinuses, and unfortunately these areas are subject to the greatest amount of image distortion. Evaluating these areas with traditional dental x-rays is therefore limited.

2D images still have their validity. Due to the fact that the image is created in a 2D pixel configuration, detail is often quite sharp. This sharpness and clarity of certain 2D x-rays allows for the detection of some diseases and conditions e.g. caries, whilst equivalent views of MSCT and CBCT images do not due to the z-axis depth dimension causing reduced sharpness and clarity. Individually, the 2D x-rays are low in radiation (Table 1), but more often than not several images are needed to gain enough information. The combined dosage along with the quantity and quality of the images should be considered before the patient is irradiated.

The quantity of information in MSCT and CBCT scans compared to conventional x-rays is very different. In order to view 3D information, the raw DICOM data needs to be loaded into a 3rd party software program. The image can then be viewed slice-by-slice using cross-sectioning tools in Axial, Sagittal and Coronal planes. Conventional dental x-rays produce images on film, paper, or computer screen. The image cannot be sectioned or viewed in any other plane other than the one it is taken in.

Subsequently reconstruction software is applied on the cone beam CT volumetric data to produce a stack of 2D grey scale level images of the anatomy.

The compact size and relatively low radiation dosage of the Cone Beam CT scanner makes it ideally suited for imaging the craniofacial region, including dental structures. With the increasing accessibility of Cone Beam CT imaging, this modality is emerging as the imaging "standard of care" for the number of diagnostic assessments of the bony components of the face.